The present invention relates to a weft yarn feeling device for shuttleless looms--of the projectile, or gripper, or fluid jet type--wherein the weft yarn, unwound by being drawn from a fixed spool or bobbin, is intermittently fed to the weaving machine at a uniform and constant tension by a device storing a weft yarn reserve (weft feeder), which is usually controlled by an electric motor with speed variable according to the amount of yarn required by the loom.
Weft yarn feeling devices (or, more simply, weft-feelers) are already normally provided on modern weaving machines, to indicate yarn breakages at the inlet of the loom.
The weft yarn may break in the area between the weft feeder and the loom, while the fabric is being formed, or in the area between the yarn feeding spool and the weft feeder.
In the first case, the weft-feeler stops the loom but cannot obviously prevent the insertion of the broken weft into the fabric: this calls for a subsequent search of the broken weft, with a waste of time and the possibility of permanent damage to the fabric.
In the second case, it is instead possible to prevent the length of broken yarn from being inserted into the fabric, by arranging a weft-feeler at the inlet of the weft feeder (or, rather, between the yarn feeding spool and the weft feeder). On breaking of the yarn, the weft-feeler stops in fact the loom when a certain amount of yarn reserve is still present on the weft feeder drum; this prevents insertion of the broken weft into the fabric. By using a weft-feeler, it will thus simply be necessary to thread again the weft feeder after yarn breakage, thereby gaining time and improving the quality of the fabric.
In the weaving practice it is already known to use, on each weft yarn, a device for feeling and detecting yarn breakage in either of the two aforementioned critical positions, that is:
between the weft feeder and the loom, as described for example in the U.S. Pat. No. 4,051,871; PA1 between the bobbin or spool and the weft feeder, as described for example in U.S. Pat. No. 4,326,564.
The present invention concerns this second type of weft-feeler, essentially consisting of a sensor detecting the presence of yarn, and of an electronic circuit which processes the signal from said sensor.
The sensor of the weft-feeler can detect the yarn motion if it is of the piezoelectric type, or the yarn tension if it is of the mechanical type with microswitch or with Hall effect, or simply the yarn presence if it is of the capacitive or optoelectric type. Other types of sensors can also be used.
It should be noted that in all cases the sensor, whatever type it may be, is not adapted to distinguish whether the yarn is broken or simply motionless or loose, as happens when the weft feeder is not drawing yarn from the spool. Thus, the signal from the sensor, indicating the presence of yarn, should be considered valid only when the weft feeder is moving.
Furthermore, on starting the weft feeder, since the yarn does not instantly reach the right speed or tension or position on the weft-feeler, the sensor may detect a false lack of yarn and needlessly stop the loom.
To prevent this from occurring, it is already known to accept the loom stopping signal only after a certain delay time, said delay having to be adjusted according to the weaving conditions, that is, according to the yarn drawing speed from the spool, which depends on many factors as, for example, the fabric width, the loom speed, the weft insertion program. It should also be borne in mind that too short delay times can cause stopping of the loom for false lack of yarn, and viceversa, that too long delay times cause stopping of the loom too long after the breakage, when all the reserve of the weft feeder is by then exhausted and the broken yarn end has thus been inserted into the fabric, destroying the advantages of the weft-feeler.
From the above it appears evident that the adjustment of the delay time is a critical operation and that, since it also depends on the program of weft insertion into the loom, it involves the need to accept arrangements which may limit the functionality of the device.
It may happen, for instance, that the first step of a program of weft insertion into the fabric, carried out on a loom fed with multicolored weft yarns, may provide for the same weft yarn to be inserted for many consecutive beatings up, thus requiring the choice of a sufficiently short delay time in order to stop the loom before exhausting the yarn reserve. A second step of the weft insertion program may provide for a far shorter frequency, thereby requiring a longer delay time in order to avoid false stopping of the loom.
Even in the absence of such difficulties, the operator still needs to adjust the delay time according to the working characteristics of the loom. This operation adds up to the many already required in order to obtain the highest efficiency of the machine, and it becomes particularly difficult when operating on looms using different colors, wherein a weft feeder and thus also a weft-feeler are provided for each color of weft.